Barium titanate (BaTiO3) has a high dielectric constant and a large piezoelectric coefficient (about 50 times that of a quartz crystal). The disadvantage is that the curie temperature is low (120 ° C), temperature stability and mechanical strength are not as good as quartz crystal.
Lead zirconate titanate is a solid solution Pb(Zr, Ti)O3 which is composed of PbTiO3 and PbZrO3. Compared with barium titanate, the pzt ceramic tube coefficient is larger, and the Curie temperature is above 300 °C. The electromechanical parameters are less affected by temperature and the time stability is good. In addition, the addition of one or two other trace elements (such as ruthenium, osmium, tin, manganese, tungsten, etc.) to zirconium titanate can also obtain PZT materials with different properties. Therefore, lead zirconate titanate piezoelectric ceramics are the most widely used piezoelectric materials in piezoelectric sensors.
Lead-free piezoelectric ceramics and their transducer shapes
(3) Piezoelectric polymer Polytetrafluoroethylene (PVF2) is a polymer film with strong piezoelectric effect. The synthetic polymer film has no piezoelectric effect in terms of its symmetry, but they have a "planar sawtooth" structure. A dipole that can't be offset. After stretching and stretching, the molecular chain axes can be regularly arranged, and spontaneously polarized dipoles are generated in a direction perpendicular to the molecular axis. When a DC high voltage electric field is polarized in the film thickness direction, it can be a polymer film having piezoelectric properties. This film is flexible and easily fabricated into a large-area piezoelectric ultrasonic tranducer. This type of component is resistant to impact, it is not easily broken, it has good stability, and has a wide frequency band. In order to improve its piezoelectric properties, it is also possible to incorporate a piezoelectric ceramic powder to form a hybrid composite material (PVF2-PZT).
(4) Piezoelectric semiconductor materials Such as ZnO, CdS, ZnO, CdTe, such a force sensitive device has the advantages of high sensitivity and short response time. In addition, ZnO is used as the piezoelectric material of the surface acoustic wave oscillator, and parameters such as force and temperature can be measured.
Measuring circuit of piezoelectric sensor
The piezoelectric sensor can be equivalent to a voltage source as shown in (a) or equivalent to a charge source, as shown in Figure (b).
voltage source and charge source equivalent circuit of piezoelectric sensor
When the piezoelectric sensor is connected to the measuring circuit, the capacitance Cc of the connecting line, the input resistance Ri of the amplifying circuit, the input capacitance Ci and the internal resistance Ra of the piezoelectric sensor should also be considered.
Voltage amplifier
The PZT material piezoelectric plates piezoelectric sensor itself has a high internal impedance and a small output energy, so its measurement circuit usually needs to be connected to a high input impedance preamplifier.
1 voltage amplifier (impedance converter) 2 charge amplifier, charge amplifier is an amplifier whose output voltage is proportional to the amount of input charge.
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