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Characteristic Property Studies of Piezoelectric Ceramics Polarization

Views:4     Author:Site Editor     Publish Time: 2019-10-31      Origin:Site

Polarization and dielectric parameters of dielectrics piezoelectric ceramics crystals are mostly dielectric crystals, also known as dielectrics. Under the action of an external electric field, the dielectric will react to the external electric field in an inductive manner. A certain amount of induced charge appears in the body or on the surface. This phenomenon is called polarization. The polarization is represented by a macroscopic polarization vector. which is equal to the vector sum of the electric dipole moments per unit volume. If the equal-numbered charge q leaves a distance l under the action of an electric field, then q represents the electric dipole moment of the charge system, and the direction is from a negative charge to a positive charge. The essence of the polarization of the dielectric in the external electric field is that the charge constituting the dielectric has a macroscopic displacement under the action of the external electric field. The positive charge will be displaced along the power line, and the negative charge will shift the reverse power line, causing the dielectric to generate a macro-electric dipole moment. Within a certain range, the polarization P is proportional to the external electric field E, I9 = £ xE, and z is called the polarization rate. From the microscopic mechanism analysis, there are three ways to generate dielectric polarization, namely electron displacement polarization, ion displacement polarization and orientation polarization of polar molecules. Regardless of the polarization actually occurring, which result can be attributed to the formation of an electric dipole in the medium, which can be characterized by the electric dipole moment of the molecule or atom. The size is determined not only by the macroscopic electric field , but also by the electric field generated by the adjacent molecules. The sum of the piezo ceramic crystals are called the effective field E. In the formula tz: a is the polarizability of a molecule or atom, which is a microscopic physical quantity describing the polarization characteristics of a molecule.

Electron displacement polarization
Under the action of an external electric field, the electron cloud in the atoms and ions that make up the dielectric will be distorted, causing the electron cloud to move relative to the nucleus, and thus the model and the circular orbit model are calculated.

Ion shift polarization
Under the action of an external electric field, the positive and negative ions in the ionic dielectric generate relative displacement, so that the dielectric generates a macroscopic electric dipole moment. This polarization is called ion displacement polarization. There is a strong interaction between positive and negative ions with a certain displacement, they will vibrate around the center of mass during equilibrium, so they can be treated as a harmonic oscillator. The ion displacement polarizability is represented by a, and the positive and negative ion masses are excellent, ∞ is its natural resonant frequency, there is P (m1 + 2)


Orientation polarization of intrinsic electric dipole moment
If the molecule constituting the dielectric is a polar molecule whose positive charge center does not coincide with the center of the negative charge, it has an inherent electric dipole moment. In the absence of an external electric field, since the electric dipole moment of the thermal motion dielectric molecules is spatially disordered, the probability of pointing in all directions is the same, and the molecular electric dipole moments each other . Therefore, the dielectric as a whole has no electric dipole moment. When an external electric field is applied, the positive and negative charges of the molecular electric dipole are affected by the electric field force, and there is a tendency to point toward the direction of the external electric field, or they must be kept in a stable state, so that the energy of the system is minimized, and it is necessary to point to the direction of the external electric field. Or precess around an external electric field. According to statistical theory, the number of particles in energy .According to this, the polarizability a of the orientation  of the buy piezoelectric crystal can be calculated (the molecular intrinsic electric dipole moment, k is the boltzmann constant, The total polarizability of the molecule can be considered as various mechanism polarization. If the number of molecules per unit volume is N, the macroscopic polarization vector P can be related to the microscopic molecular polarizability a. P = NaEP = eo (e - 1) E = NaE. Therefore, the relative dielectric constant. The effective electric field E perceived by each molecular polarization in a 1£0E medium is different from the macroscopic average electric field E. For a molecule, it is not only affected by E, but also It is affected by the electric field generated by other polarizations. The degree of response to the change of the external field during the polarization of the medium is represented by the relaxation time. The physical meaning of r is to add a constant electric field to the dielectric.After the stabilization is stabilized, the electric field is removed. After the time r, the polarization P (the sum of the electric dipole moment vectors per unit volume) is reduced to 1/e of the original P, that is, because there is a relaxation phenomenon during the polarization, D (Displacement vector), the changes in P and E are not in phase. D, P will lag behind the phase of E. The sinusoidal alternating electric field is represented by a complex number. A buzzer made of piezoelectric ceramic is placed between two circular electrode sheets, and a sinusoidal voltage of an angular frequency is applied to the electrode sheet, and the capacitance C and the admittance y of the electrode are For C.the real part of the admittance is expressed by the reciprocal of the equivalent resistance scale, but the alternating current conductivity related to polarization. The expression of y indicates that the capacitor filling the sample is equivalent to the parallel connection of the capacitance rCo and the resistance R. Let the amplitude of the sinusoidal voltage from the signal source be the reverberating current through the capacitor. In the complex amplitude voltage =, at the time of =0, the current is the real part of the work in the polarization process, and the imaginary part is not Work, the imaginary part  of the relative permittivity reflects the loss of energy during the polarization process. Defining tan as the loss factor, the dielectric parameters e, e and tan are related to the frequency and temperature of the field. When the temperature is constant, the dielectric parameters change with the change of frequency. The relationship between the two is called the dielectric frequency spectrum. .

2 Dielectric frequency spectrum analysis of piezoelectric ceramics
The three polarization modes can play different roles in different dielectrics, some of which are mainly one type and others are secondary. A guess can be made on the basis of this theory: the same dielectric, when the external electric field is an alternating electric field, produces an electric dipole moment. This polarization is called the displacement polarization of electrons. Electron displacement polarization is a form of polarization that all dielectrics have. The displacement polarization of an electron indicates that due to the influence of the external electric field, the electron will have a certain probability of absorbing energy and transitioning between the corresponding energy levels. Since the outer electrons are weakly bound by atoms, the electron displacement of atoms is mainly derived from valence electrons. The displacement polarization of piezoelectric tube piezoelectric transducer is expressed by the definition given by the model according to the three polarization modes using a point-charged spherical negative shell. Because it will change with the change of the external field, and gradually decrease as the frequency of the applied electric field increases.  it is known that e is proportional to a, so e will also decrease as the frequency of the external field becomes larger. In order to verify the theoretical guess, the dielectric spectrum of the piezoelectric ceramic buzzer is measured by dielectric spectrometer. It can be seen that when the sinusoidal alternating electric field is applied, the value of e is indeed ignored. The law of guessing decreases as the frequency increases. There is an approximately flat portion in the middle of the curve, which seems to be contrary to the guess. However, if it is considered that as the frequency of the external electric field increases, the orientation of the intrinsic electric dipole moment is slow and cannot keep up with the change of the electric field, which is reflected in the continuous decrease of the ad value. If the ad value may be as small as negligible, then a is just C . Because of the piezoelectric ceramic structure itself, the infrared frequency region is far greater than the relatively low frequency, so it is not affected by a large effect. At this time, e naturally presents a stable segment, and the curve shows that the falling slope is approximately zero. As the frequency continues to increase, crn begins to decrease significantly, and on the curve, the slope of the decline is large. To confirm our analysis of the map, we can refer to other samples to verify theoretical guesses. The dielectric frequency spectrum of the polyvinyl chloride film shows that the experimental results are in agreement with the theoretical guesses from the measured dielectric frequency spectrum curve.

3 Further improvement of the experiment
Although this experiment shows some characteristics of piezoelectric ceramic polarization on the curve, due to the limitations of the experimental main equipment DP-5 type dielectric spectrometer, it can only measure the s0 value change of l0 to 10 HZ, which is only measurement. From the long wave to a short part of the radio wave, the change of the mouth, and most of the ceramic ad changes mainly occur in the microwave and infrared regions, and the peak of the loss factor tan0 also appears in the high frequency part, The relationship between the relaxation time r and the applied field frequency 09 is known, which indicates that the internal structure of the piezoelectric ceramic and its piezoelectric characteristics cannot be further understood by using the DP-5 type dielectric spectrometer. For this reason, a dielectric spectrometer with a larger frequency range should be used to measure the piezoelectric ceramic in the ultra-low frequency, high frequency to obtain the value of e, thereby making the characteristics of the piezoelectric ceramic more and deeper. analysis.

As a new physical and chemical material, piezoelectric ceramics has important application significance in the fields of electronic engineering, materials engineering and sound and light. The study of piezoelectric ceramic dielectric spectrum can have a deeper understanding of its piezoelectricity, internal structure and polarization, especially its polarization mode. The research model for the polarization characteristics of matter used in this experiment and the prediction of images are worthy of further promotion and application in future experimental research.

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