dielectric

[diàn jiè zhì]

Physics terminology

Collection

zero Useful+1

zero

This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

A dielectric is an insulator that can be electrodeled. The charged particles of the dielectric are tightly bound by the internal forces of atoms and molecules or the forces between molecules, so the charges of these particles are bound charges. Under the action of the external electric field, these charges can only move in the microscopic range, resulting in polarization. In the electrostatic field, there can be an electric field inside the dielectric, which is the basic difference between the dielectric and the conductor.

Chinese name: dielectric
Foreign name: Dielectric

Classification: Solid, liquid, others
Discipline: Physics

catalog

nine polarization
ten application

change

Announce

edit

The positive negative charge Offset each other, macroscopic It does not show electricity, but it is outside electric field Three types of changes can be generated under the action of:

① Nucleus Extraneous Electronic cloud The distribution is distorted, resulting in Electric dipole moment , called distortion polarization ；

② Molecules with original positive and negative centers coincident, outside electric field Under the action, the positive and negative electric centers are separated from each other, which is called Displacement polarization ；

③ Have inherent Electric dipole moment The original orientation of the molecule is chaotic, and the total electric dipole moment on the macro scale is equal to zero electric field Under the action of Electric dipole It tends to be in a consistent arrangement, so that the macroscopic electric dipole moment is not equal to zero, which is called turning polarization 。

Applied to Displayed liquid crystal Materials used in the application and protection of electrostatic effects, microwave dielectric materials used in stealth technology, and dielectric materials used as structural materials.

Special effects

Announce

edit

The theory and application of special effects of dielectrics constitute another aspect of dielectric physics^[1] 。 These special effects include:

① Piezoelectric effect. When some crystals are deformed due to external forces, polarization will occur, and different sign bound charges will be formed on the opposite sides, which is called piezoelectric effect. There are many kinds of piezoelectric crystals, such as quartz, potassium sodium tartrate (Rochelle salt), potassium dihydrogen phosphate (KDP), ammonium dihydrogen phosphate (ADP), barium titanate, GaAs, zinc sulfide and other semiconductors and piezoelectric ceramics. The mechanical vibration of piezoelectric crystals can be converted into electrical vibration, which is often used to manufacture crystal oscillators. Its outstanding advantage is the high stability of the oscillation frequency. In radio technology, it can be used to stabilize the frequency of high-frequency oscillation. This kind of oscillator has been widely used in quartz clocks. Piezoelectric crystals are also widely used in electroacoustic devices such as microphones and electroacoustic heads. The piezoelectricity phenomenon can be used to measure the pressure, vibration and acceleration under various conditions^[2] 。

② Electrostriction. It is the inverse effect of piezoelectric effect. Some crystals will stretch or shorten under the action of electric field, which is called electrostriction. The electrostriction effect can be used to transform electrical vibration into mechanical vibration. Transducers commonly used to generate ultrasonic waves, earphones, tweeters, etc^[2] 。

③ Electret. The dielectric that can maintain polarization for a long time after removing external electric field or external mechanical action is called electret. Electret has both piezoelectric effect and thermoelectric effect. In technology, polar polymers are mostly used as electret materials. The electret can generate a strong electric field of 30 kV/cm. The ability of electret to store charge has been used in electrostatic photography and gas filters that absorb small particles in gas^[2] 。

④ Thermoelectric effect. Crystals with macroscopic electric dipole moment caused by spontaneous polarization and large thermal expansion coefficient are called thermoelectric crystals. The thermoelectric crystal in the state of spontaneous polarization has bound charges formed by polarization on the surfaces of the positive and negative ends of the electric dipole moment, but it does not show electrification due to the adsorption of different ions in the air. When the temperature changes, the volume of the thermoelectric crystal changes significantly, leading to a significant change in the polarization intensity, destroying the surface's electrical neutrality, and the excess charges adsorbed on the surface will be released. This phenomenon is called thermoelectric effect. Both artificially polarized ferroelectrics and electrets have thermoelectric effects. Thermoelectric effect has been used in infrared detection and thermal imaging technology^[2] 。

⑤ Electrothermal effect. As the reverse effect of thermoelectric effect, the dielectric (mostly electret) with electrothermal effect is called electrothermic. Under adiabatic conditions, when the permanent polarization strength of an electric heating body is changed by means of an external electric field, its temperature will change, which is called electrothermal effect. Adiabatic depolarization can reduce the temperature, and can be used to obtain ultra-low temperature as adiabatic demagnetization (see magnetocaloric effect). Common electrothermal materials include strontium titanate ceramics and polyvinylidene fluoride (PVF) electret^[2] Body.

⑥ Electro optic effect. The effect of some isotropic transparent dielectrics becoming optical anisotropy under the action of electric field^[2] 。

⑦ Ferroelectricity. In some dielectric crystals, there are many small areas of spontaneous polarization. Each small area of spontaneous polarization is called a ferroelectric domain, and its linearity is on the order of micrometers. In the same ferroelectric domain, the orientation of each electric dipole moment is the same, and the spontaneous polarization direction of different ferroelectric domains is generally different, so the total electric dipole moment in the macro is zero. Under the external electric field, the polarization direction of each ferroelectric domain tends to be the same, and the polarization intensity P has a nonlinear relationship with the electric field intensity E. Under the repeated action of the alternating electric field with fixed peak value, the relationship curve between P and E is similar to the hysteresis loop (see ferromagnetism), which is called the hysteresis loop. The above properties are called ferroelectricity, and the dielectrics with ferroelectricity are called ferroelectrics. When the temperature rises to a certain critical value Tc, ferroelectric domains will be mutually resolved, ferroelectricity will disappear, and ferroelectrics will be transformed into ordinary paraelectric dielectrics. Tc is called ferroelectric Curie temperature. Ferroelectrics have high permittivity. Ferroelectrics must have both piezoelectricity and thermoelectricity^[2] 。

⑧ Ferroelasticity. Some crystals can form small areas of spontaneous strain in their interior, which are called ferroelastic domains. The direction (domain state) of spontaneous strain in the same ferroelastic domain is the same, and the domain states of any two ferroelastic domains are the same or mirror symmetric. The applied stress can make the ferroelastic domain transition from one domain state to another. When the external stress changes, the strain lags behind the stress change, and the relationship between stress and strain is nonlinear. Under the action of periodic external stress, the relationship curve between strain and stress is similar to the hysteresis loop, which is called the force hysteresis loop. The above properties are called ferroelasticity, and the dielectrics with ferroelasticity are called ferroelastic bodies. The permittivity, refractive index, conductivity, thermal expansion coefficient, thermal conductivity, elastic modulus and electrostriction rate of the ferroelastic body vary with the direction, and this directivity will change with the stress. Using these characteristics has broad application prospects in manufacturing force sensitive devices^[1-2] 。

classification

Announce

edit

1. Equivalent positive center and equivalent negative center of molecule:

All dielectrics are composed of molecules and atoms. The electric effect of all positive charges in each molecule on the outside can be equivalent to the effect of the equivalent point charge concentrated at a certain point. The position of the equivalent point charge is called the positive center of the molecule; Similarly, the electrical effect of all negative charges in each molecule on the outside can be equivalent to the effect of the equivalent point charge concentrated at a certain point, and the position of the equivalent point charge is called the negative point center of the molecule;

2. Polar molecule dielectric:

Dielectric in which the equivalent positive center of each molecule does not coincide with the equivalent negative center; The positive center and negative center can be replaced by the same amount of different sign charge respectively, and there is a relative displacement between them, so that the electrical effect of each molecule on the outside can be equivalent to the effect of an electric dipole.

3. Electrodeless molecular dielectrics:

A dielectric in which the equivalent positive center of each molecule coincides with the equivalent negative center.

It can be considered that the positive charge q of each molecule is concentrated at a point, which is called the "center of gravity" of the positive charge, and the negative charge - q is concentrated at a point, which is called the "center of gravity" of the positive and negative charges; Define the vector path from the center of gravity of the negative charge to the center of gravity of the positive charge as, then the molecule can form an electric dipole^[3] 。

Electrical structure

Announce

edit

(1) The electron is tightly bound by the nucleus;

(2) In the electrostatic field, the positive and negative charges in the dielectric neutral molecules only produce microscopic relative motion;

(3) When the electrostatic field interacts with the dielectric, the dielectric molecule is simplified as an electric dipole. The dielectric consists of a large number of tiny electric dipoles;

(4) Dielectric in external electric field → polarization → polarization charge → additional electric field → acting on dielectric → achieving electrostatic balance^[3-4] 。

difference

Announce

edit

In electricians, it is generally believed that materials with resistivity exceeding 10 Ω/cm are classified as dielectrics. The charged particles of the dielectric are tightly bound by the internal forces of atoms and molecules or the forces between molecules, so the charges of these particles are bound charges. Under the action of external electric field, these charges can only move in the microscopic range to produce polarization 。 stay electrostatic field The electric field can exist in the dielectric, which is the basic difference between dielectric and conductor.

Common

Announce

edit

Common dielectrics in electricians include:

Gas dielectrics include air, hydrogen Sulfur hexafluoride (SF6)，

Liquid dielectrics are transformer oil , oil, pure water,

Solid dielectrics are mica , porcelain rubber , paper, polystyrene.

Electric dipole

Announce

edit

Dielectric is applied electric field When the is medium, the Electric dipole 。 Electric dipole It refers to that two symbols which are close to each other but have a distance are opposite but have the same value charge 。 For example, set hydrogen atom Put it in a place provided by an external power supply electric field Medium, if the external electric field is zero charge The distribution is spherically symmetric, positive negative charge The average position of Electric dipole 。 If any electric field The electric field will negative charge Pull down to positive charge Push upward, the average position of positive charge and negative charge will no longer coincide Electric dipole (See figure). Electric dipole Around it electric field 。 Its characteristics can be Electric dipole moment P means, p=qd. Here q is for each charge Electricity （ absolute value ）; The magnitude of d is equal to the distance between two charges, and its direction is specified by negative charge Point to positive charge.

polarization

Announce

edit

dielectric

The phenomenon that the vector sum of the electric dipole moment in the dielectric is not zero. Dielectric can be divided into two categories: one is right and wrong Polarity Dielectric negative charge The other is polar dielectrics (under normal conditions, the average positions of positive and negative charges of molecules in the medium do not coincide). Without electric field Equivalent of non-polar dielectric molecules when acting Electric dipole moment Is zero; Due to the disordered arrangement of polar dielectric molecules, the vector sum of their equivalent electric dipole moments is also zero. Outside electric field When acting, the positive of non-polar dielectric molecule negative charge Average position relative displacement Electric dipole moment Steering occurs. In this way polarization Phenomenon. polarization Degree of, available Electric polarization intensity P means. P is the electric dipole moment per unit volume, that is, it is a vector whose unit is International System of Units The middle is coulomb/m2. According to experiments, many dielectric Electric polarization intensity P and electric field intensity E is proportional, i.e

dielectric

Where ε 0 is Vacuum permittivity ； χ by Electropolarizability For isotropic dielectric scalar For anisotropic dielectric tensor 。

Some dielectrics have strong dipole intermolecular interaction electric field In a small volume, molecules are arranged parallel to each other, forming a macro dipole moment Of Electric domain 。 This is nothing electric field Hour Electric domain Internal molecule has appeared polarization The phenomenon of Spontaneous polarization 。 Pyroelectric material 、 Ferroelectric material All Spontaneous polarization 。 Of course, this kind of dielectrics with domain structure, due to the disordered arrangement between domains, have no electric field It also does not show polarization 。 Electric field intensity of electric displacement multiplied by Vacuum permittivity The resultant vector added with the electrode strength is the potential shift D

dielectric

Or expressed as the constitutive equation D of the dielectric= ε In formula E ε Is the dielectric constant of the dielectric. according to Gaussian Flux theorem

dielectric

This shows that Electric displacement The flux of D is determined by Free charge Qf. Although the bound charge may affect the distribution of D, it will not emit the flux of D. In some cases, it is more convenient to use this formula, because this formula Equal sign The right term does not contain bound charges. stay Time-varying electromagnetic field The time change rate of electric displacement is Displacement current Density. The unit of electric displacement is International System of Units Where is coulomb/m2 (C/m2).

application

Announce

edit

stay Electrotechnics Medium, dielectric is mainly used as electrical insulation Materials, so dielectrics are also called electrical insulating materials. With the development of science and technology, it is found that some dielectrics have Polarization process Relevant special performance. If not Symmetrical center The crystal dielectric of polarization , i.e Piezoelectricity ； There is no symmetry center, but there is a unique polar crystal with different directions Spontaneous polarization When temperature change can cause polarization Pyroelectric Sex; When the spontaneous polarization dipole moment energy is applied electric field Its polarization intensity is related to the applied electric field curve And Ferromagnetic material Of Magnetization And magnetic field The relationship curve of is very similar, that is, it has hysteresis loop ( Ferroelectricity )。 have Piezoelectricity 、 Pyroelectricity 、 Ferroelectricity Are called piezoelectric materials Pyroelectric material Ferroelectric materials. These materials with special properties are collectively referred to as functional materials. It is an important part of the dielectric. It can be used for conversion between machinery, heat, sound, light and electricity, and has extremely important applications in national defense, detection, communication and other fields.

Novice on the road

Growth task Getting Started with Editing Edit Rule Edit by myself