Piezoelectric ceramic characteristics analysis

 piezoelectric effect and dielectric effect of piezoelectric ceramic

The piezoelectric effect is that when some dielectrics are deformed by an external force in a certain direction, polarization occurs inside, and positive and negative opposite charges appear on the two opposite surfaces thereof. When the external force is removed, it will return to the uncharged state. This phenomenon is called on the positive piezoelectric effect. When the direction of the force changes, the polarity of the charge also changes. On the contrary, when an electric field is applied in the polarization direction of the dielectric, these dielectrics are also deformed, and the deformation of the dielectric disappears after the electric field is removed. This phenomenon is called an inverse piezoelectric effect or electrostriction. One type of sensor developed based on the dielectric piezoelectric effect is called a piezoelectric crystals sensor.

Any medium in the electric field will cause deformation of the medium due to the effect of induced polarization, and this deformation is different from the deformation caused by the inverse piezoelectric effect. The dielectric may be elastically deformed by an external force, piezoelectric ceramic knock sensor may be deformed by the polarization of the external electric field.The deformation due to the induced polarization is proportional to the square of the external electric field, which is an electrostrictive effect. The deformation it produces is independent of the direction of the external electric field. The deformation caused by the inverse piezoelectric effect is proportional to the external electric field, and when the electric field is reversed, the deformation also changes (for example, the original elongation can be shortened, or the original shortening can be changed to elongation). In addition, the electrostrictive effect is present in the all dielectrics, whether non-piezoelectric or piezoelectric has only the electrostrictive effects of dielectric crystals of different structures . The inverse piezoelectric effect is only found in piezoelectric ceramics crystals.

A PZT materials piezo ceramic crystal that produces a piezoelectric effect is called a piezoelectric crystal. One type of piezoelectric crystal is a single crystal such as quartz (SiO2), sodium potassium tartrate (also known as Loser salt, NaKC4H4O6.H2O), bismuth ruthenate (Bi12GeO20). Another type of piezoelectric crystal is called on piezoelectric ceramics, such as barium titanate (BaTiO3), lead zirconate titanate Pb(ZrxTirx)O3, lead bismuth magnesium zirconate titanate made in Japan, added to PZT, bismuth manganese made in China. Lead zirconate titanate Pb(Mn1/2Sb2/3)O3 was added to PIT.

Dielectric is an insulator that can be electrodeized. The use of dielectrics is quite extensive. The dielectric conductivity of piezoelectric ceramic element is very low, coupled with the good dielectric strength properties, which can be used to make electrical insulators. In addition, the dielectric can be highly electrodeposited and is an excellent capacitor material. The study of dielectric properties involves the storage and dissipation of electrical and magnetic energy within the material. This study is extremely important for explaining the various phenomena of electronics, optics and solid state physics. Dielectric properties refer to the properties of storage and loss of electrostatic energy under the action of an electric field, usually expressed by dielectric constant and dielectric loss. When high-frequency technology is applied to materials, such as solid wood composite flooring, dielectric properties are very important when high-frequency hot pressing is used. When the medium is applied with an electric field, an induced charge is generated to weaken the electric field. The ratio of the original applied electric field (in vacuum) to the electric field in the final medium is the permittivity, also known as the induced current rate.

In electromagnetism, when an electric field of piezoelectric button discs is applied to a dielectric, an electric dipole is generated due to the relative displacement of the positive and negative charges inside the dielectric. This phenomenon is called on electric polarization. The applied electric field may be an external electric field or an electric field generated by a free charge embedded inside the dielectric. The electric dipole generated by the polarization is called "inductive electric dipole", and its electric dipole moment is called on inductive electric dipole moment. Piezo ceramic is having an electrode forming ability under the action of an electric field. Divided into electrical insulation, capacitors, piezoelectric, pyroelectric and ferroelectric ceramics according to their use and performance.

Polarization of piezoelectric ceramic dielectric

Piezoelectric ceramics crystals are both dielectric and anisotropic dielectrics, so the dielectric properties of piezoelectric crystals are different from those of isotropic dielectrics.

The dielectric is polarized under the action of an electric field, and the polarization state is a state in which the electric field exerts a relative displacement force on the charge point of the dielectric and a temporary balance of mutual attraction between the charges. The electric field is the external cause of polarization. The internal cause of polarization lies in the interior of the medium. With the microscopic processes inside the medium, there are three main mechanisms of polarization.

(1) An atom or ion that constitutes a dielectric. Under the action of an electric field, a positively charged nucleus does not coincide with the negative center of its shell electron, thereby generating an electric dipole moment. This polarization is called electron displacement polarization.

(2) The positive and negative ions that make up the dielectrics undergo relative displacement under the action of an electric field, resulting in an electric dipole moment called ion displacement polarization.

(3) The molecules that make up the dielectric are polar molecules with a certain intrinsic electric moment, but due to thermal motion, the orientation is disordered, and the total electric moment of the entire dielectric is zero. When an external electric field acts, these electric dipole moments will be aligned along the outer field,ultrasound piezoelectric crystal is producing a macroscopic electric dipole moment in the dielectric, which is called on orientation polarization.

1. Displacement polarization of an infinite molecule

When the electrodeless dielectric is in an external electric field under the action of the electric field force, the positive and negative charge centers of the molecule will produce relative displacements to form an electric dipole, and their equivalent electric dipole moments P are oriented along the direction of the electric field. For a dielectric piezoelectric as a whole, since each molecule in the dielectric forms electric dipoles, they are arranged in the dielectric. The positive and negative charges of adjacent electric dipoles in the dielectric are close to each other. If the dielectric is uniform, it remains electrically neutral throughout it, but on the surface of the dielectric that is perpendicular to the external electric field strength E0. There will be positive and negative charges, respectively, which cannot leave the dielectric and cannot move freely in the dielectric, This phenomenon of polarized charges in the dielectric under the action of an external electric field is called polarization of the dielectric. The stronger external electric field, the larger the relative displacement between the positive and negative charge centers of each molecule, the larger is the electric dipole moment of the molecule, the more polarized charges appear on both surfaces of the dielectric, and the more polarized high. When the external electric field of resonance frequency piezoelectric transducer is removed, the centers of the positive and negative charges are again coincident (P = 0), so this type of molecule can be regarded as an elastic electric dipole whose elastic force is connected by two equivalent equivalent electric charges. The magnitude of the electric dipole moment P is proportional to the field strength. Since the polarization of the infinite molecule lies in the relative displacement of the center of the positive and negative charges, it is often called a bit.

Oriented polarization of polar molecules

As for the polar molecular dielectric, the center of the positive and negative charges in the molecule is equivalent to an electric dipole. Under the action of the external electric field, it will be subjected to a moment, so that the electric dipole moment P of the molecule is turned to the direction of the electric field. Because of the interference of molecular thermal motion, this steering is tiny, and it is impossible to align the electric dipole moments of all molecules along the direction of the electric field. The stronger external electric field of piezoelectric electrode piezoelectric ceramic, the more tidy is the steering order of the electric dipole moment of the molecule. At the macroscopic level, the more polarized charges appear on both surfaces perpendicular to the dielectric and the external electric field, the higher degree of polarization. When the external electric field is removed, the direction of the electric dipole moment of the molecule becomes an irregular arrangement due to the thermal motion of the molecules, and the dielectric is still neutral. Polarization of polar molecules lies in the direction in which the equivalent electric dipole turns to the external electric field, so it is called orientation polarization. In general, while the molecules are polarized at the same time, there is also displacement polarization. Although the microscopic processes of polarization of two types of dielectrics, polar is different, but the macroscopic effects are the same. Polarized charges of different numbers piezoelectric plate sensors appear on the two opposite surfaces of the dielectric, and the external electric field increases. the more polarized charges appear. Therefore, when the polarization phenomenon of the dielectric is described macroscopically below, it is not necessary to divide into two kinds of dielectrics for discussion.

3.      Ferroelectricity of piezoelectric ceramic crystals

The polarization of some dielectrics is very special. In a certain temperature range, their dielectric constants are not constant, but vary with field strength, and after removing the external electric field, these dielectrics are not neutral. There is residual polarization. In order to be analogous to the fact that ferromagnetic materials can remain magnetized, this property of piezo ceramic transducer is often referred to as ferroelectricity. A ferroelectric dielectric is called a ferroelectric. Among them, barium titanate ceramics (BaTiO3), sodium potassium tartrate single crystal (NaKC4H4O6⋅H2O) and the like are most prominent.Ferroelectrics will exhibit hysteresis during the electrodeposition process. The hysteresis loop shows that the polarization between the ferroelectric body and the applied electric field is nonlinear, and the polarization is reversed as the external electric field is reversed. The polarization inversion is the result of domain inversion, so the hysteresis loop indicates the presence of domains in the ferroelectric. The so-called domains are small regions in which the spontaneous polarization directions in the ferroelectrics are uniform, and the domains. The boundary between them is called the domain wall. Ferroelectric crystals of piezoelectric ceramic products are usually multi-domains, the spontaneous polarization in each domain has the same direction, and the spontaneous polarization in the different domains is strong.

For polycrystalline ferroelectrics, there is no regularity between the relative orientations of spontaneous polarization in the different domains for the entire polycrystal due to the complete arbitrariness of the orientation of the crystal axes between the grains.

Ferroelectrics generally do not spontaneously form single domains, but multidomain crystals can be monodomainized under a strong external electric field. Under the action of a strong external electric field, the domain volume of the spontaneous polarization in the multi-domain crystal parallel or close to the external field direction will rapidly expand due to the formation of new domain nuclei and the movement of the domain walls, and the domain volume in other directions will decrease rapidly. Small disappears,which is turning the entire crystal into a single domain. Under the action of external electric field, the dynamic process of new domain nucleus and domain wall motion is called the domain reversal process. This reversal has some hysteresis characteristics, so the ferroelectric exhibits the aforementioned hysteresis loop.

Considering a single piezo crystal is assuming that the orientation of spontaneous polarization has only two possibilities.it is positive and negative along a certain crystal axis; the direction of the external electric field is parallel to the polarization axis. When the external electric field is zero, the polarization of adjacent domains in the crystal is opposite, and the total electric moment of the crystal is zero. When the external electric field is gradually increased, the domain volume of the spontaneous polarization direction opposite to the direction of the electric field will gradually decrease due to the inversion of the domain, and those domains are having the same direction as the electric field will gradually expand, so that the crystal is in the direction of the external field. The intensity increases with the increasing of the electric field. When the electric field of piezoelectric disc element increases enough to reverse all the reverse domains in the crystal to the external field, the crystal becomes a single domain, the polarization of the crystal reaches saturation, and then the electric field increases. The polarization will increase linearly with the electric field (same as the polarization of a typical dielectric) and reach a maximum value Pmax, which is a function of the highest polarization electric field. When the linear portion is extrapolated to zero electric field, the resulting intercept Ps on the vertical axis is called the saturated polarization, which is actually the spontaneous polarization of each domain.When the electric field begins to decrease from C, the polarization will gradually decrease along the C-B curve. When the electric field of piezoelectric ceramic component is reduced to zero, the polarization decreases to a certain value Pr, which is called the residual polarization of the ferroelectric. When the electric field changes direction and increases to Ec in the negative direction, the polarization decreases to zero, the reverse electric field continues to increase, and the polarization is reversed. Ec is called the coercive field strength of the ferroelectric. As the reverse electric field continues to increase, the polarization continues to increase in the negative gradient direction and reaches a saturation value (-Pr) in the negative direction, and the ultrasonic piezoelectric transducer becomes a single domain crystal which is having a negative polarization. If the electric field changes continuously from a high negative value to a high positive value, the positive domain begins to form and grow again until the entire crystal becomes a single domain crystal with forward polarization again. During this process, the polarization is returned to point C along the FGH portion of the return line. Thus, under the action of a large alternating electric field, the electric field changes by one week, and the above process is repeated once, showing the hysteresis loop shown. The area enclosed by the return line is the energy required to invert the polarization twice.