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Four. Classification of piezoelectric materials
1. The first type of piezoelectric material piezoelectric single crystal .This is a naturally occurring or artificial single crystal ferroelectric material with anisotropy. Its piezoelectric effect is based on the composition of the crystal structure Caused by changes in the relative position of the positive and negative ions on the lattice. Commonly used piezoelectric single crystals .Quartz (SiO2): This is a naturally formed or artificially cultivated (man-made crystal) crystal with good uniformity and high curie point; high impedance and high mechanical Q (Qm); high hardness and good wear resistance; The performance is extremely stable, the aging is extremely slow and minimal, and its performance changes with temperature is very small, and the linear frequency temperature coefficient that does not change with time can be obtained; the loss is small,which can be used for extremely high frequencies; the insulation performance is good, it can be high use under voltage;which can be used in higher and extremely low temperature environments. Because of its many superior properties, quartz is still widely used today, especially as a standard transducer and as a time oscillator in computer equipment. Its disadvantage is that the electromechanical conversion efficiency is low, which makes the gain of the system loop lower.
Lithium Niobate (LiNbO3): This is an artificially grown ferroelectric single crystal with a diameter of up to 120mm. Lithium niobate can be used to directly excite the ultrasonic transverse wave with high electromechanical coupling coefficient and excellent piezoelectric performance. It has a large Qm value and a high curie point. It can be used at high temperatures, with stable polarization and ultrasonic propagation. The loss is small, it is not deliquescent, and the frequency constant is very large. It can be used to make ultra-high frequency transducers .Therefore, it has been used as a common basic material for surface acoustic wave transducers. When used as a volume wave transducer, it can obtain better sensitivity than common piezoelectric ceramic transducers. It is also used for ultrasonic thickness measurement ,narrow measurement and pulse transducer. This is also an artificial single crystal. It has good mechanical properties, is easy to process, can be dissolved in water but is not easy to deliquesce,which has relatively stable physical and chemical properties, and has excellent piezoelectric properties of PZT material piezoelectric strip
. The electromechanical coupling coefficient and low dielectric constant, and the Qm value are quite low, they are suitable for making high-sensitivity, high-resolution broadband transducers and delay lines, such as making ultrasonic thickness measurement and narrow pulse transducers. In addition, there is lithium sulfate (Li2SO4) with good receiving performance.
3. The second type of piezoelectric material piezoelectric ceramics .This is a polycrystalline ferroelectric material made by manual firing of powder sintering method. The piezoelectric effect is based on the electrostrictive effect, and its piezoelectric properties change with sintering. There are differences in the craftsmanship and formulation ingredients, so there are many types and different performances. For example: grinding the material to 400 mesh, adding a binder, pressing, baking at high temperature, and then sawing, grinding and polishing into a finished piezoelectric ceramic wafer. Piezoelectric ceramics are easy to be made into various shapes, and can be vibrated in a variety of vibration modes to adapt to various uses. It has high electromechanical coupling coefficient, high loop gain and sensitivity, which are its important advantages. Commonly used piezoelectric ceramics are: Barium titanate (BaTiO3): This is a mixture of titanium dioxide (TiO2) and barium carbonate (BaCO3) sintered at high temperature. This is an earlier piezoelectric ceramic, its curie temperature is low, the temperature dependence is large, and the time stability and thermal stability are poor. Now it is still used for sonar radiators and ultrasonic transducers. Lead zirconate titanate , code PZT, has a variety of formulas and characteristics, and is currently the most commonly used piezoelectric ceramics.
The main feature of the PZT series piezoelectric plate crystal is the high electromechanical coupling coefficient, of which the PZT-4 is a transmission type, and its high excitation characteristics are good (high Qm value, small internal loss, etc.), which is suitable for sonar radiators and ultrasonic transducers. , High voltage generator and high power transducer. PZT-5 is a receiver type. It has high dielectric constant, low aging, and low Qm value. It is suitable for hydrophones, ultrasonic transducers, record players, microphones, and speaker components. It is also suitable for broadband pulse type detection, etc. In addition: PZT-2, PZT-5A, PZT-5H, PZT-6A, PZT-7A, PZT-8 ... and so on.
Lead niobate system: The dielectric constant is large, the Kp is medium, and the sound frequency characteristics are good. Lead tungsten cadmium system is good temperature and time stability of frequency. Lead Magnesium Tellurate. It can withstand repeated pressure, and has low aging of electrical and mechanical properties. In addition, there are lead lithium antimonate and lead lithium tantalate, which have good stability and low Qm values, and are suitable for underwater acoustics transducer. In addition to ternary piezo ceramics, quaternary piezo ceramics of niobium nickel-niobium zinc-titanium-lead zirconate have been developed.
The third type of piezoelectric material-polar polymer piezoelectric material. This is a new artificially synthesized semi-crystalline polymer with a piezoelectric effect, called a polar polymer, and its piezoelectric effect is based on a polar polymer. The molecular rotation is currently best with polyvinylidene fluoride (PVDF). PVDF (-CH2-CF2-) is one of the most polar polymers. The PVDF film is stretched to several times its original length at a temperature below 100 ° C to obtain a β-type (a crystalline form of PVDF) film, which is applied with an electrode (usually aluminum) and polarized in a high DC electric field ( Temperature is 80-150 ℃), will obtain piezoelectric performance, it can be effectively used as an acoustic receiver, has good thermal stability, in addition, the material can be bent, the acoustic impedance is small, and it is well matched with water, especially suitable for hydrophones and transducers for medical ultrasound diagnostic sound field testing. The disadvantages of piezoelectric film materials are that the signal-to-noise ratio is not ideal, the electromechanical coupling coefficient is not large enough, and the mechanical and dielectric losses are relatively large. In addition, because the quality factor (Qm, Qe) is small, it is not suitable for places where sharp resonance is required, nor for large input and continuous work, because its piezoelectric effect when used for a long time at a temperature above 80 ° C Decrease. In addition, polar polymer piezoelectric materials include polyfluoroethylene (PVF2) and the like.
The fourth type of piezoelectric material--composite piezoelectric material and zinc oxide piezoelectric film. Composite piezoelectric material energy harvesting plate is composed of ferroelectric ceramic particles dispersed and mixed in polymer materials. Like electrical materials, their piezoelectric properties depend not only on the ceramic particles, but also on the type of polymer materials used as the matrix, especially the composite systems with high-permittivity polymers such as PVDF and vinylidene fluoride. , Can be used as strong piezoelectric materials. This piezoelectric material does not need to be stretched like other polymer piezoelectric bodies, and is isotropic internally. With the change of the matrix polymer type, a large range of elastic modulus can be obtained. In particular, it can be hot-pressed and practical. very convenient. For example, the composite materials of PVDF and PZT series have very stable piezoelectric properties and dielectric properties. These materials have reached the practical stage and are similar to piezoelectric polymer materials in application.
Zinc oxide (ZnO) piezoelectric film (made by vacuum spraying process) is used for ultra-high frequency ultrasonic generation and receiving transducers. It can be used in the 30-3000 MHz frequency band and has a good effect. It can be used for the study of material properties , ultrasonic delay line, acousto-optic device, communication and information processing, and ultrasonic microscope, etc., with frequency bandwidth, good electro-acoustic conversion efficiency, and easy matching with the excitation circuit. In addition, cadmium sulfide (CdS), aluminum nitride (AlN), etc. are also good piezoelectric thin film materials.