What’s the piezoelectric material and structure ?

Views: 7 Author: Site Editor Publish Time: 2019-10-16 Origin: Site

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Piezoelectric material is a special dielectric material with piezoelectric effect and inverse piezoelectric effect. The piezoelectric effect is the characteristic of some piezo crystals discovered by the French P. Curie and J. Curie brothers in 1880. When a piezoelectric force exerts a mechanical force (or release pressure) on its piezoelectric direction, the piezoelectric body will generate a charge and discharge phenomenon. This phenomenon is called a positive piezoelectric effect.An electric field that is the same (or opposite) to the direction of polarization causes two effects: an inverse piezoelectric effect and an electrostrictive effect. The inverse piezoelectric effect, that is, the dielectric is mechanically deformed by an external electric field, and the magnitude of the strain is proportional to the magnitude of the applied electric field, and the direction is related to the direction of the electric field. The electrostrictive effect, that is, the dielectric field of the Pzt material piezoelectric ceramics generate strain due to induced polarization, and the magnitude of the strain is proportional to the square of the electric field, which is independent of the direction of the electric field. The inverse piezoelectric effect and the electrostrictive effect are essentially the result of polarization of the dielectric crystal under the action of an external electric field, and the lattice is distorted, and the macroscopically appears as mechanical strain.

Piezoelectric ceramics are piezoelectric ceramics obtained by mixing ingredients, sintering at a high temperature, and having a piezoelectric assembly after a solid phase reaction between the particles. PZT material can be used as both a sensing element and a driving element, and can be embedded with other materials to form a composite material. Therefore, it has a wide range of applications, such as aircraft handling on aircraft wings, and in vibration systems. Active control of vibration and noise for structural health monitoring in equipment.

The main features of PZT application in smart material structure are:
1 can be used as both a driver and a sensor;
2 as the driver, its excitation power is small;
3 The response speed is faster, which is 100 times of the shape memory alloy;
4 size can be made very small and thin, can be installed on the surface of the structure, or buried in the structure;
5 Combination flexibility can be used in a relatively large form, or can be used in small pieces.

2 PZT structure

PZT material is a continuous solid solution of Pbzro3 and PbTio3, which is in the ABO3 perovskite structure. Founded in the early 1950s, PZT material is a compact piezoelectric ferroelectric material with important technical application value. Piezoelectric ceramics are crystalline dielectric materials that do not have a center of symmetry. Piezo crystal dielectrics that do not have a center of symmetry have a 432-point group crystal with extremely low inverse piezoelectric effect due to extremely high symmetry. The crystal dielectric of the symmetry center is deformed by the inverse piezoelectric effect. Under the action of the electric field, the dielectric is polarized because there is no ionic bond between the sub-dissociation and the rightmost positive ion (and others). so during the polarization process, a large relative displacement can occur between them, which shows a large inverse piezoelectric effect at the macroscopic level. Expressed as: S = dE,which is proportional to the size of the electric field. That is to say, in the piezoelectric material, the electrical quantity and the mechanical quantity are coupled to each other, and the energy stored in the medium is composed of two parts, one part is strain energy and the other part is electromagnetic energy.

According to the modern structural dynamics theory, when damage and defects occur in equipment and structures, such as cracks, loose bolts, etc., its rigidity and mechanical impedance characteristics will change, which will also lead to changes in the natural frequency and mode of the structure. Therefore, the degree of damage can be quantitatively given based on the change in mechanical impedance. However, mechanical dynamic impedance varies with frequency and is difficult to measure by using conventional methods. Using the self-driven and self-sensing characteristics of the piezoelectric element, PZT material piezo round discs can simultaneously act as a driving element and a sensing element to excite the structure to obtain the dynamic response of the structure, thereby establishing a bridge between mechanical characteristics and electrical information, mechanical dynamic impedance information. Changes can be reflected by simple measured electrical information. When a certain external voltage is applied to the surface of the piezoelectric ceramic sheet, a lateral surface force is generated on the surface of the beam. These surface forces will drive the beam to produce different vibrations (when the upper and lower PZT are subjected to the same voltage, the beam will vibrate longitudinally; when a reverse voltage is applied, the beam will be subjected to bending vibrations). In turn, the vibration causes the beam to deform, and the deformation characteristics can be reflected in the form of electrical signals through the sensing characteristics of the piezoelectric ceramic sheets. Therefore, the dynamic admittance characteristics of the piezoelectric ceramic sheets pasted on the structure can reflect the damage state of the structure. According to the piezoelectric coupling effect, and the interaction between PZT and the structure, the frequency-dependent admittance (the reciprocal of the impedance) can be obtained. When the parameters and performance of the PZT remain be constant, the structural impedance uniquely determines the value of the second term. Any change of piezoelectric sodium corresponds to structural damage and defects, so that structural damage can be identified by the value of piezoelectric sodium.

Implementation of PZT piezo for structural health monitoring

Because of the piezoelectric effect and the inverse piezoelectric effect of the piezoelectric element, the piezoelectric element has dual functions of driving and sensing, and this feature can realize on-line and real-time health monitoring of the structure.

A part of the PZT material is connected to the power source generating the excitation signal through a wire, and an excitation signal (voltage or charge) is applied to the PZT material through a voltage or a charge driving power source, because the PZT material has an inverse piezoelectric effect, that is, a deformation occurs under the action of an electric field due to The PZT material is embedded (or adhered to) the base material, so its own deformation will be transmitted to the base material, and the base material will be deformed or moved together. At this time, the PZT is equivalent to a driver, and the deformation is generated by receiving the excitation signal. Or exercise to drive the base material. At the same time, some PZT material is placed on the base material and is not connected to the power source, and this deformation or motion is transmitted to the PZT material when the base material is deformed or moved. Due to the piezoelectric effect of the PZT material, a charge is generated inside the charge, and the magnitude of the charge varies with the magnitude of the deformation or motion. At this time, the PZT material is equivalent to a sensor. Then, the output signal of the PZT sensor is measured and collected by the measuring device, and the deformation or movement of the base material can be reflected in real time and online, thereby realizing real-time and online health monitoring of the structure.

Comparing the data collected in the real time with the vibration data when the structure is normal, to see if the PZT material output signal changes (such as cracks or looseness of the structure, theoretically will cause the output of the PZT piezoelectric ceramics transducer in the structure to change, if it changes, it is considered this structure has failed. When a fault occurs, the signal can be transmitted to the controller in time to timely deal with the structural failure to realize online, real-time monitoring, fault diagnosis and fault handling of the structure.

The PZT material can simultaneously act as a drive and sensing element to excite the structure to obtain the dynamic response of the structure. The dynamic response relationship between piezoelectric ceramic sheets and external structures was analyzed by the principle of positive and negative piezoelectric effects. When the external structure changes, the corresponding piezoelectric impedance also changes. By measuring the admittance of the piezoelectric system, the state of the structure can be predicted . PZT material is suitable for both macroscopic damage and micro-damage, and has good prospects for structural health monitoring of buildings in the future.