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Piezo effect and piezoelectric acoustic sensor

Views: 20     Author: Site Editor     Publish Time: 2020-03-26      Origin: Site

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Acoustic emission signals are propagating in the solid media contain characteristic information of acoustic emission sources. To use this information to reflect material characteristics or the development of defects, it is necessary to receive such acoustic emission signals on solid surfaces. Acoustic emission signals are transient random wave signals with a minimum vertical displacement of about 10-7,10-14 meters. The frequency is distributed in the infrasound to ultrasonic frequency range (several hertz to tens of megahertz). This requires that acoustic emission detection instruments have high response speed, high sensitivity, high gain, wide dynamic range, strong blocking recovery capability, and frequency detection of piezo ceramic disc crystal can be selected. In the actual acoustic emission detection process, the detected signals are often complex signals that have undergone multiple reflections and waveform transformations. Acoustic emission signals are received by sensors and converted into electrical signals. The sensors give a frequency-sensitivity curve according to a specific calibration method. Based on this, different types of sensors with different frequencies and sensitivities can be selected according to the purpose and environment of detection.


The ultrasonic sensor is made based on the principle that the physical characteristics of certain substances (such as semiconductors, piezo ceramics, piezoelectric crystals, ferromagnetic bodies, and superconductors) change with the outside to be measured. It uses many effects (including physical effects, chemical effects, and biological effects) and physical phenomena, such as the use of piezo resistance, moisture sensitivity, heat sensitivity, light sensitivity, magnetic sensitivity, and gas sensitivity of materials. Displacement, magnetic field, gas is measured and converted into electricity. The discovery and utilization of new principles, new effects, and the development and application of new physical materials have led to the development of physical sensors. It is therefore necessary to understand the various effects on which the sensors are based, and to understand, develop and apply them. In the acoustic emission detection process, the piezoelectric effect is usually used.


The piezoelectric effect is reversible, which is a general term for the positive and negative piezoelectric effects. It is customary to call the positive piezoelectric effect a piezoelectric effect.


When certain dielectric materials of Pzt material piezoelectric ceramic are deformed by external force in a certain direction, positive and negative charges of different signs are generated on certain surfaces of the dielectric material. After the external force is removed, they return to an uncharged state. This phenomenon is called positive piezoelectric effect. The charge generated by a dielectric force is proportional to the magnitude of the external force. The proportionality coefficient is a piezoelectric constant, which is related to the direction of mechanical deformation. It is constant for a certain direction of a certain material. The polarity of the charge generated by a dielectric force depends on the form of deformation (compression or elongation). Materials with significant piezoelectric effects are called piezoelectric materials. Commonly used are single crystals such as quartz crystals, lithium niobate LiNbO3, lithium gallate LiGaO3, bismuth germanate Bi12GeO20, and polycrystals such as barium titanate after polarization treatment. Electric piezo ceramics, lead zirconate titanate series piezoelectric ceramics PZT. New piezoelectric materials include polymer piezoelectric films (such as polyvinylidene fluoride PVDF) and piezoelectric semiconductors (such as ZnO, CdS). The piezoelectric effect of single crystal materials is due to the deformation of the internal meridian structure of these single crystals when subjected to external stress,which is causing the original macroscopically neutral state to be destroyed and resulting in polarization. The piezoelectric ceramics and polymer piezoelectric films after polarization (a strong electric field is applied at a certain temperature) is the result of the polarization of electric domains and electrode pairs.


Piezoelectric sensors are made using the positive piezoelectric effect convert non-electrical energy such as pressure, vibration, acceleration, etc. electrical energy is used for precise measurement.


When an electric field is applied in the direction of polarization dielectric of Pzt8 piezoceramic ring componnets, some dielectrics will produce mechanical deformation or mechanical stress in a certain direction. When the external electric field is removed, the deformation or stress also disappears. This physical phenomenon is called the inverse piezoelectric effect. Utilizing the inverse piezoelectric effect can be made into ultrasonic generators, piezoelectric speakers, piezo crystal oscillators with a highly stable frequency (such as quartz clocks and watches with a day and night error ). The inverse piezoelectric effect can be used for acoustic emission signal generation. Because piezoelectric conversion elements have two important properties of self-generating and reversible, plus its small size, light weight, simple structure, reliable operation, high natural frequency, high sensitivity and high signal-to-noise ratio, the application of piezoelectric sensors get rapid development. A variety of voltage generators such as piezoelectric power supplies, gas stoves, and auto-ignition devices for automobile engines developed using the positive piezoelectric effect in the testing technology, the piezoelectric conversion element is a typical force-sensing element, which can measure the final changeable .Those physical quantities that form a force, such as pressure, acceleration, mechanical shock, and vibration,  therefore, the application of piezoelectric sensors can be seen in a wide range of fields such as acoustics, mechanics, medicine, and aerospace. What is more significant is that according to the results of bio-piezoelectrics, it is recognized that living things have piezoelectricity, and various human sense organs are actually bio-piezoelectric sensors. If the fracture is treated according to the positive piezoelectric effect, the healing can be accelerated; the inverse piezoelectric effect can be used to power the bones to correct deformed bones and other functions. The main disadvantage of piezoelectric conversion elements is that they have no static output, which require high electrical output impedance, and require low-capacitance and low-noise cables. The operating temperature of many piezoelectric materials is only about 250 ° C.


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