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Piezoelectric material piezoceramics are crystalline materials that generate a voltage between their two ends when pressure is applied. In 1880, French physicists P. Curie and J. Curie brothers discovered that when a heavy object is placed on a quartz crystal, some surfaces of the crystal will generate charges, and the amount of charge is proportional to the pressure. This phenomenon is called the piezoelectric effect. Immediately afterwards, the Curie brothers discovered the inverse piezoelectric effect, that is, the piezoelectric body deforms under the action of an external electric field. The mechanism of the piezoelectric effect is: the crystal with piezoelectricity has low symmetry. When it is deformed by an external force, the relative displacement of the positive and negative ions in the unit cell makes the positive and negative charge centers no longer overlap, resulting in macroscopic polarization of the crystal, while The surface charge surface density of the crystal is equal to the projection of the polarization intensity on the surface normal direction, so when the piezoelectric material is deformed by pressure, charges of opposite signs will appear on the two ends of the piezoelectric material. Conversely, when a piezoelectric material is polarized in an electric field, the material deforms due to the displacement of the charge center.
Piezoelectric plate sensor can generate electric fields due to mechanical deformation, and can also generate mechanical deformation due to the action of electric fields. This inherent electromechanical coupling effect makes piezoelectric materials widely used in engineering. For example, piezoelectric materials have been used to make smart structures. In addition to self-supporting capabilities, such structures also have functions such as self-diagnosis, self-adaptation, and self-healing, and will play an important role in future aircraft design.
Technical parameters of piezoelectric materials:
Piezoelectric coefficient d33
Piezoelectric coefficient is the conversion coefficient of piezoelectric ceramics crystal that converts mechanical energy into electrical energy or converts electrical energy into mechanical energy, reflecting the coupling relationship between the elastic properties and dielectric properties of piezoelectric materials
Free permittivity εT33 (free permittivity)
The permittivity of a dielectric at zero (or constant) strain, expressed in Farads/meter.
Relative permittivity εTr3 (relative permittivity)
The ratio of the dielectric constant εT33 to the vacuum dielectric constant ε0, εTr3=εT33/ε0, is a dimensionless physical quantity.
Dielectric loss (dielectric loss)
The dielectric is the energy lost in the dielectric due to the electric polarization relaxation process and leakage conduction under the action of the electric field.
Loss angle tangent tgδ (tangent of loss angle)
Under the action of a sinusoidal alternating electric field, the current flowing in an ideal dielectric is 90 0 ahead of the voltage phase, but in the piezoelectric ceramic sample, due to energy loss, the phase angle ψ of the current lead is less than 900, and its complementary angle δ (δ+ψ =900) is called the loss angle, which is a dimensionless physical quantity. People usually use the loss tangent tgδ to represent the size of the dielectric loss, which represents the ratio of the active power (loss power) P of the dielectric to the reactive power Q . That is: electrical quality factor Qe (electrical quality factor)
The value of the electrical quality factor is equal to the reciprocal of the loss tangent value of the sample, expressed by Qe, which is a dimensionless physical quantity. If the parallel equivalent circuit is used to represent the piezoelectric ceramic sample in the alternating electric field, then Qe=1/ tgδ=ωCR
Mechanical quality factor Qm (mechanical quality factor)
The ratio of the mechanical energy stored by the piezoelectric plate vibrator at resonance to the mechanical energy lost in one cycle is called the mechanical quality factor. The relationship between it and the oscillator parameters is: Poissons ratio
Poisson's ratio refers to the ratio of lateral relative shrinkage and longitudinal relative elongation of a solid under stress, and is a dimensionless physical quantity expressed by δ: δ= - S 12 /S11
Series resonance frequency fs (series resonance frequency)
The resonant frequency of the series branch in the equivalent circuit of the piezoelectric vibrator is called the series resonant frequency, expressed by f s ,
Parallel resonance frequency fp (parallel resonance frequency)
The resonant frequency of the parallel branch in the equivalent circuit of the piezoelectric vibrator is called the parallel resonant frequency, represented by f p, that is, f p = resonant frequency fr (resonance frequency)
The lower frequency of a pair of frequencies that makes the susceptance of the piezoelectric vibrator zero is called the resonant frequency, represented by f r .
Antiresonance frequency fa (antiresonance frequency)
The higher frequency of a pair of frequencies that makes the susceptance of the piezoelectric vibrator zero is called the anti-resonance frequency, expressed by f a .
Maximum admittance frequency fm (maximum admittance frequency)
The frequency when the admittance of the piezoelectric vibrator is large is called the large admittance frequency. At this time, the impedance of the vibrator is small, so it is also called the small impedance frequency, expressed by f m.
Small admittance frequency fn (minimum admittance frequency)
The frequency at which the admittance of the piezoelectric vibrator is small is called the small admittance frequency. At this time, the impedance of the vibrator is large, so it is also called the large impedance frequency, expressed by f n.
fundamental frequency
The low resonant frequency in a given vibration mode is called the pitch frequency, and usually becomes the fundamental frequency.
Overtone frequency (fundamental frequency)
The resonant frequencies other than the fundamental frequency in a given vibration mode are called overtone frequencies.
temperature stability
Temperature stability refers to the characteristic that the performance of piezoelectric ceramics changes with temperature.
At a certain temperature, when the temperature changes by 1 °C, the ratio of the numerical change of a certain frequency to the numerical value of the frequency at this temperature is called the temperature coefficient of frequency TKf.
In addition, a large relative drift is usually used to characterize the temperature stability of a certain parameter.
Relative frequency shift at positive temperature=△f s (large positive temperature)/f s(25℃)
Large relative frequency shift at negative temperature=△f s (large negative temperature)/f s(25℃)
Electromechanical coupling coefficient (ELECTRO MECHANICAL COUPLING COEFFICIENT)
The electromechanical coupling coefficient K is the square root of the ratio of the elastic-dielectric interaction energy density square V122 to the product of the stored elastic energy density V1 and the dielectric energy density V2.
Piezoelectric ceramics commonly use the following five basic coupling coefficients
A. Plane electromechanical coupling coefficient KP (reflects the polarization and electric excitation of the thin disc along the thickness direction, and is a parameter of the electromechanical coupling effect during radial stretching vibration.)
B. Transverse electromechanical coupling coefficient K31 (parameters reflecting the electromechanical coupling effect of the slender strip along the thickness direction polarization and electrical excitation for length stretching vibration.)
C. Longitudinal electromechanical coupling coefficient K33 (a parameter reflecting the electromechanical coupling effect of the thin rod along the length direction of polarization and electrical excitation for length stretching vibration.)
D. The electromechanical coupling coefficient KT of thickness stretching
E. Thickness shear electromechanical coupling coefficient K15 (reflects the polarization of the rectangular plate along the length direction, the direction of the excitation electric field is perpendicular to the polarization direction, and is used as a parameter for the electromechanical coupling effect during thickness shear vibration.)
Piezoelectric strain constant D (PIEZOELECTRIC STRAIN CONSTANT)
The piezoelectric strain constant is the ratio of the change of the strain component SI to the change of EI caused by the change of the electric field component E under the condition that both the stress T and the electric field component EM (M≠I) are constant.
Piezoelectric voltage constant G (PIEZOELECTRIC VOLTAGE CONSTANT)
The constant is the ratio of the change of the electric field intensity component EI caused by the change of the stress component TI to the change of TI under the condition that the electric displacement D and the stress component TN (N≠I) are both constant.
Curie temperature TC (CURIE TEMPERATURE)
Piezoelectric ceramics only have a piezoelectric effect within a certain temperature range. It has a critical temperature TC. When the temperature is higher than TC, the piezoelectric ceramic undergoes a structural phase transition. This critical temperature TC is called the Curie temperature.
Temperature stability (TEMPERATURE STABILITY)
Refers to the characteristics of the performance of piezoelectric ceramic transducers that change with temperature. Generally, there are two methods for describing temperature stability: temperature coefficient or large relative drift.
Ten times aging rate (AGEING RATE PER DECADE) Y represents a certain parameter
Frequency constant (FREQUENCY CONSTANT)
For the radial and transverse length-stretching vibration modes, the frequency constant is the product of the series resonant frequency and the element dimension (diameter or length) that determines this frequency. For longitudinal length thickness and stretching shear vibration modes, the frequency constant is the product of the parallel resonance frequency and the vibrator size (length or thickness) that determines this frequency, and its unit: HZ.M
