Views:1 Author:Site Editor Publish Time: 2019-10-11 Origin:Site
Piezoelectric ceramics have been widely used due to their piezoelectricity and the resulting diversity of electromechanical properties. These applications can generally be divided into two broad categories, namely as piezoelectric vibrators. When is used as a piezoelectric vibrator, the piezoelectric ceramic material is required to have good frequency temperature stability and a high mechanical quality factor Q (Q indicates the degree of internal energy consumption of the material during vibration conversion); it is required to be used as a transducer. The high mechanical coupling factor K (mechanical conversion to electrical energy / input mechanical energy, electrical energy to mechanical energy / input electrical energy) and large relative dielectric constant, the application of piezoelectric ceramics is given below.
Piezoelectric ceramic is a polycrystalline film with piezoelectric effect, and its production process is named after its similar production process (raw material pulverization, molding, high temperature sintering). Some anisotropic crystals undergo deformation under mechanical force, causing the charging particles to be relatively displaced, resulting in the positive and negative bound charges on the surface of the crystal. This phenomenon is called piezoelectric effect. This property of the crystal is called piezoelectricity. Piezoelectricity ceramics was discovered in 1880 by J. Curie and P. Curie brothers. A few months later they experimentally verified the inverse piezoelectric effect, that is, when a voltage is applied to the piezo crystal, the piezo crystal will undergo geometric deformation. Before 1940, only two types of ferroelectrics were known (not only spontaneously polarized in a certain temperature range, but also the spontaneous polarization of the crystals that can be reoriented due to the external field strength): one is potassium dihydrogen phosphate and its equivalent. The former has piezoelectricity at normal temperature, and has technical usage value, but has the disadvantage of being easy to dissolve; the latter has piezoelectricity ceramics at low temperature (less than -14 C), and the engineering use value is not large. Barium titanate (BaTiO) was found to have an abnormally high dielectric constant . It was soon found to be piezoelectric, and the discovery of BaTi O piezoelectric ceramics was a quantum leap for the piezoelectric materials. Previously, there was only a piezoelectric crystal material, and thereafter a piezoelectric polycrystalline material, piezoelectric ceramic appeared and was widely used. In 1947, the United States used BaTiO ceramics to make pickups for phonographs. Japan used it two years. BaTiO material has the disadvantage that the piezoelectricity is weaker than the resting salt and the piezoelectricity is larger than the quartz crystal with temperature. In 1954, B. Jaffe and others discovered the piezoelectric PbZrO-PbTiO (PZT) solid solution system, which is an epoch-making event that made it impossible to fabricate devices in the BaTiO . Since then, PZT transparent piezoelectric ceramics have been developed to extend the application of piezoelectric ceramics to the field of optics. So far, the application of piezoelectric ceramics, from the development of the universe to the life of the family, is extremely extensive. China's research on piezoelectric ceramics began in the late 1950s, about 10 years later than foreign countries. At present, there are quite strong forces in the trial production and industrial production of piezoelectric ceramics. Many materials have reached or are close to the international level.
The physical mechanism of piezoceramic piezoelectricity
Piezoelectric ceramics are polycrystals whose piezoelectricity piezo disc sensor can be explained by the piezoelectricity of the piezo crystal. Under the action of mechanical force, the total electric dipole moment (polarization) changes, resulting in a piezoelectric phenomenon. Piezoelectricity is closely related to polarization, deformation, and the like.
Microscopic mechanism of polarization
The polarization state is a state in which the electric field exerts a relative displacement force on the charged point of the dielectric and a temporary balance of mutual attraction between the charges. There are three main polarization mechanisms.
(1) Electron displacement polarization—The atom or ion of a dielectric does not coincide with the negative charge center of a positively charged nucleus and a shell electron under the action of an electric field force.
(2) Ion displacement polarization—the positive and negative ions of the dielectric are relatively displaced under the action of an electric field force, thereby generating an electric dipole moment.
(3) Orientation polarization—the polar molecules that make up the dielectric have a certain intrinsic (inherent) electric moment. Due to thermal motion, the orientation is disordered, the total electric moment is zero. When an electric field is applied, the electric dipole moment .The direction of the electric field is aligned and a macroscopic electric dipole moment appears.For anisotropic crystals, the polarization is related to the presence of an electric field.
2. Piezoelectric effect
(1) Positive piezoelectric effect
When the piezoelectric crystal is deformed by an external force, the positive and negative charge centers are relatively displaced, and the opposite charges are generated on some corresponding faces, and the polarization intensity occurs. This phenomenon of no electric field and polarization by deformation is called a positive piezoelectric effect.
For anisotropic crystals, stress is applied to the crystal; (corresponding strain), the crystal will have a proportional polarization in the three directions of X, Y, and Z, which are called piezoelectric stress constant and piezoelectric strain constant, respectively.
(2) Inverse piezoelectric effect
When an electric field is applied to the crystal, not only polarization but also deformation is generated, and this phenomenon of deformation by the electric field is called an inverse piezoelectric effect. This is because when the crystal is subjected to an electric field, stress (piezoelectric stress) is generated inside the crystal, and piezoelectric strain is generated by stress.
3. Mechanism of pressure effect
The piezoelectric effect was first discovered on piezo crystals. Now we use piezo crystals as a model to illustrate the physical mechanism of the piezoelectric effect.
When no pressure is applied, the positive and negative charge centers of the crystal are distributed. At this time, the positive and negative charge centers coincide, and the total electric moment of the crystal is equal to zero, and the crystal surface is not charged (not piezoelectric).
Application of piezoelectric ceramics
Piezoelectric ceramic igniter
This is a device that converts mechanical force into an electric spark to ignite the combustion, and is an electromechanical transducer. In 1958, the piezoelectric effect of barium titanate (BaTiO) ceramics was used for ignition. However, PZT material has a low ignition rate and high noise. In 1962, trials of lead zirconate titanate (PZT) piezoelectric ceramics were used to make igniters. Igniters are widely used in the daily life, industrial production, and military applications to ignite gas and various types of explosives and rockets.
(1) Basic principles
High-voltage generation——Taking a cylindrical piezoelectric ceramic component as an example, when the mechanical force F acts on the cylinder, the piezo crystal is distorted, causing the center of the positive and negative charges in the crystal to shift, so that a large amount of charge accumulates on the upper and lower surfaces of the cylinder and a
(2)high voltage output.
(3)igniter structure and working principle
2. Piezoelectric transformer
Since the 1950s, piezoelectric transformers have been developed. At that time, barium titanate was used as the main material. The boost is relatively low (only 50-60 times). The output voltage is around 3000 volts. With the advent of lead zirconate titanate piezoelectric ceramic materials, the boost ratio has been increased to 300-500 times, and it has been gradually applied to televisions, electrostatic copiers, and negative ion generators as high-voltage power supplies.
(1) Basic principles
The electrical vibration energy input to the piezoelectric ceramic sheet is converted into mechanical vibration energy by the inverse piezoelectric effect, and then converted into electric energy by the positive piezoelectric effect. Impedance conversion (from low impedance to high impedance) is achieved in these two energy conversions to achieve a high voltage output at the resonant frequency of the piezo ceramic chip. The transformation principle is explained by taking a horizontal and vertical transformer of stretching vibration as an example.
The whole piezo ceramic piece is divided into two parts, the left part is the input end (also called the driving part), the upper and lower sides have the infiltrated silver electrode, which is polarized in the thickness direction, and the right part is the output end (also called the power generating part), and the right end.There is a silver electrode that is infiltrated on the surface. Polarized along the length. When the input terminal is applied with an alternating voltage, due to the inverse piezoelectric effect, the piezo ceramic sheet generates stretching vibration along the length direction, which converts the input electrical energy into mechanical energy; and the power generating portion converts the mechanical energy into electrical energy through the positive piezoelectric effect. Where the mechanical quality factor of the material; - the longitudinal and transverse electromechanical coupling coefficients of the material; the length of the L - power generating portion; T - the thickness of the transformer.
(2) Application of piezoelectric transformer
Piezoelectric transformers are mainly used in the case of high voltage, low power and sine wave conversion, and have the unique advantages of high output voltage, light weight, small volume, no leakage magnetic field, no combustion. In order to obtain multiple voltage outputs, according to the output voltage of the horizontal-vertical transformer is proportional to the length, the closer to the end of the power generation part, the higher the voltage, we can make electrodes as taps at different positions of the power generation part, thus obtaining the different voltage outputs. .
4.Piezoelectric ceramic pickups and speakers
(1) Double diaphragm type vibrator
The working principle of the double diaphragm type vibrator is given. When a piezoelectric ceramic with a certain thickness is bent under the force, it is elongated on the one side of the thickness and compressed on the other side, and a charge is generated inside the piezo ceramic sheet. However, since the entire diaphragm has the same polarization direction, the upper side is elongated, and the lower side is compressed, so that the electric dipole moment is opposite, and the upper and lower side charge symbols are the same, so there is no potential difference, such as switching to two overlapping double diaphragm structure, when is subjected to force bending, the voltage output can be obtained. Two pieces of diaphragms with opposite polarization directions are connected in the series, and when the force is applied, the upper piece is elongated and the lower piece is compressed. Since the polarization directions are opposite, the upper and lower sides of the double diaphragm are oppositely charged with a sign, and a voltage output can be obtained. The two diaphragms with the same polarization direction are connected in parallel to form an output voltage.
(2) Piezoelectric ceramic pickup structure and working principle
(3) Piezoelectric ceramic speaker structure and working principle
Piezoelectric ceramic speaker is a simple and light-weight electroacoustic device, which has the advantages of high sensitivity, no magnetic field scattering, no copper wire and magnet, low cost, low power consumption, convenient repair and mass production.
The driving system is a PZT material piezoelectric elements double diaphragm, the vibration system is a paper cone, and the coupling component transmits the energy of the driving system to the vibration system efficiently. During operation, the electrical energy applied to the piezoelectric ceramic double diaphragm is converted into mechanical energy, which is transmitted to the paper cone through the coupling element to vibrate and sound. The piezoelectric double diaphragm has a higher impedance and constitutes a voltage drive. The relationship between the force F and the voltage V is F=KV, K is a proportional coefficient, and the vibration mechanical impedance including the radiation impedance is Z, and the vibration speed is
The sound pressure P at the center r of the high vibration film can be obtained.
In addition, other electro-acoustic energy converters such as a transmitter, a receiver, a buzzer, etc. can be made according to the piezoelectric effect of the piezoelectric ceramic.
(4) Piezoelectric ceramic fans and relays
The piezoelectric ceramic can be made into a small piezoelectric ceramic fan, which has the advantages of small volume,there is no heat generation, no hum, low power consumption and long life. It is a piezoelectric ceramic bending deformer, which is composed of two piezoelectric ceramic sheets sandwiched by a metal foil, and the piezo ceramic sheet generates a telescopic movement under the action of an external electric field. If two piezo ceramic sheets are applied with a reverse voltage, the other side is contracted to stretch, and the metal sheet is bent and deformed. If an alternating voltage is applied, the metal sheet will periodically vibrate.