what's the characteristics of piezoelectric ceramic material

what's the characteristics of piezoelectric ceramic material

    Judging whether piezoelectric ceramic materials are suitable for surface acoustic wave devices can be considered in the following aspects:

    (1) The dielectric constant should be moderate. Generally, the dielectric constant is required to be 10 to 1000. Too large a dielectric constant will cause direct coupling between the interdigital electrodes. If it is too small, the impedance is too large, and the connection with the circuit and the device is not easy to match. In the piezoelectric ceramics of the monolithic system, except for the dielectric constant of BaTiO3 of 1,700, both PbTiO3 and PbNb2O6 are small, about 200, which can be used as a piezoelectric material for higher frequency surface acoustic wave devices. Other binary and multi-element piezoelectric ceramic materials have a dielectric constant of 200 to 1000, which is in line with the moderate dielectric constant.

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    (2) It is possible to have a good surface suitable for the manufacture of the interdigital electrodes by processing. Generally, the single crystal material is very dense, and the surface is ideal after subsequent processing such as cutting and polishing. Piezoelectric ceramics are generally made up of powdered materials of different compositions. After a series of treatments, it is sintered at a high temperature, so its grain size and pore size are the main indicators of piezoelectric ceramic materials for surface acoustic wave devices. It not only determines the smoothness of the material, but also determines the upper operating frequency of the device. As noted above, the operating frequency of the surface acoustic wave device depends on the inter-finger bar width of the interdigital transducer, which requires the surface air hole dimension to be at least less than the inter-finger width of the interdigital transducer. For example, a surface acoustic wave bandpass filter with a center frequency of 60 MHz is fabricated on the surface of a piezoelectric ceramic material having a surface acoustic wave velocity of 2400 m/s. Its surface acoustic wave wavelength A is one second / factory, so A = 40 μm. The interdigitated strip width is λ/4, which is 10 μm. In order to reduce the reflection of sound waves, a split-finger transducer is adopted, that is, each finger is λ/8, and each finger strip is only 51 um wide. In order to prevent the influence of material pores and grains on the metal fork fingers, the pore size and grain size are required to be at least less than 3 μm. Therefore, in order to obtain a good surface suitable for the manufacture of the interdigital electrodes, the crystal grains and pores of the piezoelectric ceramic material are required to be as small as possible.

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    (3) The surface acoustic wave transmission attenuation is small. The attenuation of the surface acoustic wave transmission is related to the physical properties and surface state of the piezoelectric ceramic material itself. If the pores and grains are too large, scattering loss will occur, and the friction loss during vibration between the grains will also cause attenuation. Therefore, in addition to the processing, it is determined by selecting materials.

    (4) It is necessary to have the highest possible electromechanical coupling coefficient to improve the electromechanical transducing efficiency. The electromechanical coupling coefficient reflects the efficiency of the mutual conversion between the mechanical energy of the piezoelectric material and the electric enthalpy, and it is a very important indicator. It is not only closely related to the elastic properties, dielectric properties and piezoelectric properties of materials, but also closely related to different vibration modes. In order to improve the conversion efficiency, the larger the electromechanical coupling coefficient is, the better, which can also reduce the energy loss during signal processing. Generally, piezoelectric ceramic materials have relatively large electromechanical coupling coefficients, so this is easy for piezoelectric ceramics.

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    (5) The temperature characteristics are better. In order to ensure the stability of the device, the temperature characteristics of the material are good, and the frequency aging rate is small. In particular, the temperature coefficient of the surface acoustic wave delay should be as small as possible. This is because once the interdigital finger is determined, its operating frequency is also set, and the main design basis for the operating frequency and the interdigital width is the surface acoustic wave delay time (ie, the speed of sound). Therefore, as the time delay changes, the operating frequency changes, which will cause the device to not work properly. The TCD is generally required to be less than 25 x 10*-6/°C.

    (6) It is not easy to age. The general requirement is 0.5%/year or less, and the strict requirement is 0.1%/year or less.

    (7) Consistency and repeatability are better. When the device is mass-produced, the performance difference between the same material or the local area of the same batch of material is small, so that the device can work normally. This has little effect on the piezoelectric single crystal because the uniformity and repeatability of the piezoelectric single crystal material are better, and the piezoelectric ceramic material has a greater influence. The main factors affecting the dispersibility of materials are raw material dispersion, batch weighing deviation, sintering temperature and time control. When the deviation of key characteristics such as the speed of sound reaches 1%, it is unacceptable for the mass production of the parts, and therefore it is strictly required to be less than 0.1%. The so-called repeatability generally refers to the degree of characteristic deviation between different batches, and the consistency generally refers to the degree of characteristic deviation between the same batch or the same batch of sinter. It is difficult to separate the two. If the consistency is not good, there will be no good repeatability. This problem is a prominent weakness for piezoelectric ceramics. When piezoelectric ceramic materials are used as acoustic body vibrators or transducer components, frequency modulation or sorting measures are often required to compensate for this weakness. When used as a substrate for a surface acoustic wave device, since it is not adjustable, it must have good repeatability and consistency, otherwise it will not be applicable even if other characteristics are good.