Piezoelectric effect and piezoelectric acoustic emission sensor

The sensor is based on the principle that the physical properties of certain substances (such as semiconductors, ceramics, piezoelectric crystals, ferromagnets, and superconductors, etc.) change as the outside is to be measured. It takes advantage of many effects (including physical, chemical, and biological effects) and physical phenomena, such as the use of piezoresistive, moisture sensitive, heat-sensitive, photo-sensitive, magneto-sensitive and gas-sensitive effects of materials, strain, humidity, temperature, Displacement, magnetic fields, gas, etc. are measured and converted into electricity. The discovery and use of new principles, new effects, and the development and application of new materials have led to great development in physical sensors. Therefore, it is necessary to understand the various effects based on the sensor and to understand, develop, and apply it. In the acoustic emission detection process, the piezoelectric effect is usually used.

Piezoelectric effect is reversible, it is a collective term of positive piezoelectric effect and inverse piezoelectric effect. It is customary to refer to positive piezoelectric effects as piezoelectric effects.
When some dielectrics are deformed by external forces along a certain direction, positive and negative sign charges are generated on a certain number of surfaces. When the external force is removed, it returns to the uncharged state. This phenomenon is called positive. Piezoelectric effect. The electric charge produced by the dielectric force is proportional to the magnitude of the external force. The proportional coefficient is the piezoelectric constant. It is related to the direction of mechanical deformation, and it is constant to a certain direction of a certain material. The polarity of the charge generated by the dielectric depends on the form of the deformation (compression or elongation).
The material with obvious piezoelectric effect is called piezoelectric material, commonly used are quartz crystal, lithium niobate LiNbO3, lithium niobate LiGaO3, niobium bismuth Bi12GeO20 and Other single crystals and polarised polycrystals such as barium titanate Electric ceramic, lead zirconate titanate series piezoelectric ceramic PZT. New piezoelectric materials include piezoelectric polymer films (such as polyvinylidene fluoride PVDF) and piezoelectric semiconductors (such as ZnO and CdS). The piezoelectric effect of a single-crystal material is due to the deformation of the internal lattice structure of these single crystals when subjected to external stress, which results in the destruction of the electrically neutral state of the original macroscopic appearance and the generation of electrical polarization. The piezoelectricity of piezoelectric ceramics and polymer piezoelectric films after polarization (a strong electric field is applied at a certain temperature) is the result of polarization of electric domains and electrode dipoles.
Piezoelectric sensors made with positive piezoelectric effects convert non-electrical quantities such as pressure, vibration, and acceleration into electrical quantities for precise measurements.
When an electric field is applied in the polarization direction of the dielectric, 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 inverse piezoelectric effect. . The use of inverse piezoelectric effect can be made into ultrasonic generators, piezoelectric speakers, crystal oscillators with high frequency stability (for example, quartz clocks and meters with errors of < 2×10-5s per day). The inverse piezoelectric effect can be used for acoustic emission signal generation.
Since the piezoelectric conversion element has two important performances of self-generation and reversible, plus its small size, light weight, simple structure, reliable operation, high natural frequency, sensitivity and high signal-to-noise ratio, therefore, the application of piezoelectric sensors Get rapid development. A variety of voltage generators such as piezoelectric power supplies, gas furnaces, and auto-ignition devices for automotive engines have been developed using positive piezoelectric effects; in the testing technology, piezoelectric transducers are a typical force-sensitive element that can be measured and eventually transformed The physical forces of Chengli, such as pressure, acceleration, mechanical shock, and vibration, can be seen in piezoelectric, sensory, medical, and aerospace applications. More importantly, according to the results of bio-piezoelectricity, it is recognized that all living things have piezoelectricity, and human's various sensory organs are actually bio-piezoelectric sensors. Such as the treatment of fractures based on positive piezoelectric effect, can accelerate recovery; the use of inverse piezoelectric effect, the power to the bone has the function of correction of deformed bone.
The main disadvantage of piezoelectric transducers is that they have no static output and require high electrical output impedance. Low-capacity, low-noise cables are required. The operating temperature of many piezoelectric materials is only about 250°C.