The basic principle of ultrasonic transducer
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Keywords : 安全使用热风循环烘箱及四个注意要点
Piezoelectric Transducers: Piezoelectric transducers utilize the piezoelectric effects of certain single crystal materials and the electrostrictive effects of certain polycrystalline materials. Ultrasonic Piezoelectric Effect The structure of some monocrystalline materials has asymmetric properties. When these materials are subjected to external stress, the internal lattice structure changes (deformation) to destroy the original state of the original macro- The resulting electric field (electrostriction) is proportional to the magnitude of the strain. This phenomenon is called the positive piezoelectric effect, which was discovered by Curie Brothers in 1880. Subsequently, in 1881 and further found that such single crystal material also has a reverse piezoelectric effect, that is, with a positive piezoelectric effect of the material by the external electric field, there will be stress and strain, the strain and the size of the external electric field into Proportional. The piezoelectric effect is a characteristic of the crystal structure, which is related to the asymmetry of the crystal structure, and the magnitude and nature of the piezoelectric effect are related to the relative direction of the applied stress or electric field to the crystal axis. There are many types of single crystal materials with piezoelectric effects, such as natural quartz (SiO2) crystals, and artificial single crystal materials such as lithium sulfate (Li2SO4), lithium niobate (LiNbO3) and so on.
Electrostrictive effect In some polycrystalline materials there is a spontaneous formation of the molecular group, the so-called "domain", it has a certain polarization, and the length of the polarization direction is often different with the length of the other direction. When there is an applied electric field, the domain will rotate and its polarization direction will be consistent with the direction of the applied electric field, so that the length of the material in the direction of the applied electric field will change, showing elastic strain. This phenomenon is called electrostrictive effect.
Magnetostrictive transducers The magnetostrictive transducers utilize the magnetostrictive effect, where the physical properties of the crystalline structure of a particular alloy material, i.e., some ferromagnets and their alloys, and the magnetic properties of certain ferrite The length of the domain in its spontaneous magnetization direction may be different from that in the other directions. When there is an external magnetic field, because of this magnetic domain will rotate, so that the direction of magnetization as far as possible with the direction of the external magnetic field tends to be consistent, so that the material along the direction of the external magnetic field will change, the performance of elastic strain (of course, This strain caused by the strain is very small, about 10-5 ~ 10-6 between). This phenomenon is the magnetostrictive effect. On the contrary, the magnetostrictive effect of the material subjected to external stress or strain, its magnetization will change, this is the inverse magnetostrictive effect. Thus, when an alternating magnetic field is applied to the magnetostrictive material, the material will undergo magneto-induced deformation in the direction of the magnetic force lines so that a mechanical vibration wave can be excited in the medium in close contact with its surface.
Likewise, the purpose of receiving the ultrasonic wave by means of the inverse magnetostrictive effect is that the strain (elastic stress-ultrasonic force) applied to the magnetostrictive material will cause the magnetic flux density of the magnetic field of the material in the applied magnetic field (The so-called magnetoelastic effect), so that the detection coil located on the surface of the material will change due to the magnetic flux density generated by the induced potential, can be used as a magnetoelastic effect of the signal to achieve the effect of receiving ultrasound (note the direction of the magnetic field Should be consistent with the direction of stress - ultrasonic vibration generated by the same direction). According to the state of magnetostrictive changes, can be divided into:
 linear magnetostrictive: in the event of strain, the volume of the material unchanged, but in the length of the direction of the degree of expansion and change, which is the main application of magnetostrictive transducer type. However, it can only occur in the Curie temperature below the case, if the temperature exceeds the Curie point will only exist in the volume type magnetostrictive.
 Volume of magnetostrictive: in the event of strain, the volume of the material will change. Magnetostrictive transducers are mainly used for low-frequency high-power applications, which is limited by frequency and limited by magnetic material characteristics of the factors, it is particularly useful in the field of power ultrasound applications, which is characterized by mechanical High strength, stable performance, low water pressure requirements (no hydrolysis). However, its vortex and hysteresis losses are greater, electro-acoustic conversion efficiency than piezoelectric transducers, and usually requires a large amount of excitation power for high power applications. It should be noted that the effect of the skin effect is limited by the influence of the skin effect when the alternating magnetic field is applied, so that the range of the magnetostrictive effect is limited to the surface of the material.
In the case of ultrasonic waves, the strength of the ultrasonic wave depends on the strength of the alternating magnetic field on the surface of the material. In addition, the tightness (acoustic coupling) of the contact between the acoustic medium and the surface of the material is also extremely important. Commonly used in the magnetostrictive transducer materials are metal nickel, metal cobalt, iron and cobalt alloy, iron-nickel alloy, nickel ferrite, nickel zinc ferrite, nickel and copper ferrite. This is a device that converts electrical energy into mechanical energy or converts electrical energy into electrical energy. The structure is shown in Figure 1.1. The cylinder connected to the diaphragm is a coil with a thin enameled wire wound around it. , The sleeve being fitted over the center magnetic conductor. Electromagnetic transducers: Electromagnetic - acoustic transducers (also known as vortex - acoustic transducers): Electromagnetism - Electromagnetism A device that produces ultrasonic waves in conductive metals. Capacitive transducers (also known as electrostatic transducers).